If the past year or two of Gauteng water trouble — burst mains, 54-hour planned outages, coliform scares that had municipalities dosing extra chlorine — pushed you into buying a tank, you’ve probably already had the hardware-store moment: the label says 25mm, your ruler says 33mm, and nothing on the shelf seems to match the hole in your tank. You’re not going mad. South African tank fittings are labelled one way and measured another, and once you understand why, buying the right part takes thirty seconds.
What thread do JoJo tank fittings use?
Almost every tank fitting sold in South Africa — JoJo’s own outlets included — uses BSP thread: British Standard Pipe. It’s an old imperial standard, which is exactly why the confusion happens. BSP sizes are named in inches (¾”, 1″, 1½” and so on), but SA retailers label them in millimetres.
Here’s the catch: the millimetre label describes the nominal bore — the hole through the middle — not the thread you can measure. A “25mm” fitting has roughly a 25mm waterway inside, but its male thread measures about 33mm across the outside. Put a ruler on a “40mm” JoJo outlet thread and you’ll read closer to 48mm.
So when a fitting “doesn’t fit” even though the numbers match, it’s almost always one of two things: you measured the thread and shopped by that number, or you picked up a metric irrigation compression fitting that was never BSP in the first place. More on that under the mistakes section.
Which fitting size do I need? The chart
Five sizes cover practically every household tank job in SA. Bookmark this table.
| Label (mm) | BSP name | Male thread OD (approx.) | Typical use |
|---|---|---|---|
| 20mm | ¾” | ±26mm | Float valves, level gauges, municipal top-up lines |
| 25mm | 1″ | ±33mm | Gravity-fed garden taps and wash-up points |
| 32mm | 1¼” | ±42mm | Pump suction feeds |
| 40mm | 1½” | ±48mm | Standard JoJo bottom outlet, tank-to-tank linking, overflows |
| 50mm | 2″ | ±60mm | High-flow setups and very large tanks |
A few notes on the common ones:
- 40mm is the big one. The standard JoJo bottom outlet is 40mm, so that’s the size for your main draw-off, your overflow, and for linking two tanks together.
- 25mm runs your taps. Gravity pressure is gentle, so a 25mm line keeps flow usable without a pump.
- 32mm feeds pumps. Typical household booster pumps (0.37–0.75kW) want a generous suction line, so don’t strangle yours with a skinny fitting.
- 20mm handles the small stuff: float valves for municipal top-up and gauge fittings.
How do I measure the fitting I already have?
Don’t guess from the label on a weathered fitting — measure it. The trick is to measure the outside of the male thread, then read the chart backwards:
- Find the male half of the joint (the part with thread on the outside).
- Measure straight across the thread at its widest point with a ruler or vernier.
- Match that number to the “male thread OD” column above. About 33mm? You need 25mm fittings. About 48mm? That’s 40mm.
For a female fitting, measure across the opening from thread to thread — it accepts a male thread of roughly that same OD. And never measure the waterway hole and shop by that; the bore is always a few millimetres off the nominal size, which is how people end up with a drawer of almost-right fittings.
Male, female, and where do the washers go?
A tank connector (also called a bulkhead fitting) is what turns a hole in a plastic tank wall into a threaded connection. The threaded body passes through the wall with a flange on one side and a backnut on the other — and this is where most leaks are born.
You need a rubber washer on both sides of the tank wall: inside AND outside. EPDM rubber is the usual choice because it handles water and sunlight without perishing. One washer seals against the water; the other lets the nut clamp evenly on a wall that’s never perfectly flat. Fit only one and you’ll get a weep that appears days later, usually after you’ve packed the tools away.
Tighten the backnut by hand, then a quarter to half turn with a spanner. Polyethylene tank walls flex — the washer does the sealing, not brute force.
The fittings most tanks need
What are the three mistakes that cause drips?
- Sealing with tape instead of washers. Tank connectors use parallel BSP threads that seal on the washer face. Wrapping thread tape around them and skipping (or reusing a flattened) washer gives you a joint that weeps forever. Tape belongs on tapered male threads screwing into sockets — not on bulkheads.
- Overtightening. Crank a plastic backnut and you crack the flange or dimple the tank wall so the washer can’t seat. If it drips after a firm quarter-turn, the problem is the washer or the surface, and more force makes it worse.
- Mixing standards. Metric compression fittings for black LDPE irrigation pipe are true-metric and will never seal on a BSP thread — even when the shelf label shows the same number. If a joint threads on skew or bottoms out after two turns, stop: you’re mating two different systems.
Honest note: fitting quality is rarely the villain. An R89 connector with two healthy EPDM washers seals as well as anything fancier — nearly every “faulty fitting” story we hear turns out to be a missing outside washer or an overtightened nut. Keep a spare washer pack in the toolbox and you’ll fix most drips in five minutes.
FAQ
Is a JoJo tank outlet 40mm or 50mm?
The standard JoJo bottom outlet is 40mm (1½” BSP) — the male thread measures about 48mm across. Some very large or high-flow installations use 50mm, so measure before you buy if the tank isn’t a standard household model.
Are tank fittings the same as irrigation fittings?
No. Irrigation compression fittings grip plain poly pipe and are sized true-metric. Tank fittings are threaded BSP. They meet only through a proper adaptor, never by forcing the threads.
Should I use thread tape or a washer?
Both — in different places. Washers seal parallel-thread joints like tank connectors and float valves. Thread tape seals tapered male threads going into female sockets, like a ball valve onto an outlet nipple. Neither substitutes for the other.
What size float valve do I need for municipal top-up?
A 20mm plastic float valve handles most household top-up lines; step up to a 25mm brass valve for faster refill and longer service life. If your tank overflows even with a float valve fitted, the valve or its washer is usually the culprit.
WaterMart is Pretoria-based and launches in August 2026, couriering nationwide at R85 flat — free over R950, all prices including VAT. Our 20–50mm BSP connectors, washer spares and full fitting kits are on the pre-launch list now: join the waitlist and be sorted before the next outage notice lands.